Method of treating the surfaces of cold-worked iron chromium alloys



Patented Aug. 11, 1 925..

UNITED STATES PATENT OFFICE.

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METHOD TREATING THE SURFACES OF COLD-WORKED IRON CHROMIUM ALLOYS.

No Drawing.

- the Surfaces of Cold-Worked Iron Chromium Alloys, of which thefollowing is a specification. I

This invention relates to chromium iron alloys and more particularly tothose alloys known as stainless iron and stainless steel. In suchalloys, the chromium content may vary between 8% and and the carboncontent may vary depending upon the particular use to, which articlesmade from the alloy are to be put. The alloy contain silicon and othermetalloids. 11 one alloy with which I havehad experience, the chromiumcontent is approximately 12% and the carbon 'is approximately 12%. Thisalloy, when annealed, is suitable for the manufacture of articles suchas pump shafts, valve stems, bolts, nuts,

screws, pole line hardware and various other machined articles. Inmaking such articles, the metal is worked coldiand by this I meanmachining operations such as the cutting or grinding of threads andother surfaces.

It has been found that when such material is worked cold,the surface isphysically distorted and does not resist corrosion to the desired andnecessary degree, unless it is polished. The polishlng of such alloys isdifficult and expensive and in many cases, practically impossible.

I have discovered that if the physically distorted surfaces of articlesmade from iron chromium alloys having a chromium content between 8% and60% are treated with a nitric acid solution, their resistance tocorrosion is greatly increased. Alloys containing between 9% and 16% ofchromium with carbon under 1% have their resistance to corrosionincreased to a marked degree by immersing them in a hot nitric acidsolution.

of between 12% and 15% and the resistance is increased even though thearticle has not been hardened, tempered and polished.

The surface of articles made from such .alloys in the annealed conditioneven though Application filed December 27, 1924'. Serial No. 758,371.

the surface has been physically distorted by cold working, can be mademore resistant to corrosion than are the surfaces of hardened andtempered articles given a commercial polish.

I am aware that such articles as sheets and plates composed of suchalloys have been pickled to remove scale and that in such pickling,nitric acid solutions have been used. So far as I am aware, however, noone has discovered howto increase the resistance to corrosion ofsurfaces that have been physically distorted by cold working except bypolishing such surfaces.

My discovery provides a simple and cheap methodof rendering suchsurfaces highly resistant to corrosion."

The method may be carried out by merely immersing the machined articlescomposed of such alloys in a bath of hot nitric acid for a few minutesand such bath will preferably be a 12% to 15% solution.

In some way not definitely understood by me, this treatment increasesthe resistance to corrosion of these physically distorted s'urbined withthe chromium. Nevertheless, the

fact remains that such treatment is efiective.

Having thus described my invention, what I claim is 1. The method ofincreasing the. resistance to corrosion of articles formed cold fromchromium iron alloys, which consists in treating the surfaces of sucharticles with a solution of nitric acid.

2. The method of increasing the resistance to corrosion of the machinedsurfaces of articles formed from chromium iron alloys, which consists intreating such surfaces with a solution of nitric acid.

3. The method of increasing the resistance to corrosion of machinedsurfaces of articles formed from chromium iron alloys having a chormiumcontent of between 8% and 60%, which consists in immersing such Itreating the surfaces of such articles with a. of nitric acid ofapproximately 12% 'or solution of nitric acid. 15%.

5. The method of increasing the resistance In testimony whereof, I havehereunto 10 to corrosion of articles formed cold from subscribed my namethis 22nd day of Deceml chromium iron alloys having a chromium ber,1924.

. content between 8% and 60%, which consists in immersing such articlesin a hot solution OTHO M. OTTE.

